1. Field of the Invention
An aspect of this disclosure relates to a video transmission apparatus.
2. Description of the Related Art
A video transmission apparatus used, for example, for a conference system typically includes an image coding unit that encodes and compresses image data captured by a camera to transmit image data or an image signal (or a video signal) with desired quality to a remote apparatus via a network with a limited bandwidth.
The image coding unit employs inter-frame prediction coding where a difference between an immediately-preceding frame and a current frame is transmitted, and motion processing where a motion compensation vector indicating a movement of a part of an image is transmitted.
A video transmission apparatus also includes a panning function for changing the direction (orientation or angle) of a camera horizontally and a tilting function for changing the direction of the camera vertically, i.e., functions to adjust the angle (orientation or direction) of the camera in response to a user operation.
When the direction of the camera is changed at a high speed with the panning and tilting functions while capturing images, changes in the images caused by the directional change of the camera are added to changes in the images caused by the movement of an object such as a person.
For this reason, a motion compensation vector detection unit of a video transmission apparatus needs to detect a movement (or a change between adjacent frames) that is a combination of a movement of a moving object (e.g., a person) and a directional change of the camera. This in turn increases the time necessary to detect an amount of movement and reduces detection efficiency. That is, when the camera is not moved, the motion compensation vector detection unit needs to search only a range that is predictable based on an immediately preceding movement of an object. On the other hand, when the camera is moved, the motion compensation vector detection unit needs to expand the search range and this increases processing time. When the search range is not wide enough, it is not possible to properly detect the amount of movement. As a result, a large error occurs in an inter-frame prediction image calculated by motion compensation, and coding efficiency is drastically reduced.
Japanese Laid-Open Patent Publication No. 06-38204 discloses an image signal transmission apparatus. In the disclosed image signal transmission apparatus, to solve the above problems, a camera movement amount signal (motion vector) indicating the direction and amount of movement of a camera is input to an image coding unit, and a motion compensation vector detection unit shifts the search range according to the direction and the amount of movement indicated by the motion vector and detects a motion compensation vector by comparing images within the shifted search range.
The related-art method using a camera movement amount signal is effective in a case where the same motion vector is applicable to the entire frame (image or screen). However, the related-art method is not suitable for a case where the same motion vector is not applicable to the entire frame (image or screen) due to the distortion and a projection method of a lens. When the same motion vector is applied without change in the latter case, a proper motion compensation vector cannot be detected. As a result, a large error occurs in an inter-frame prediction image calculated by motion compensation, and coding efficiency is drastically reduced. When the search range is expanded to cope with this problem, it is contrary to the object of the related-art method, i.e., improving the processing efficiency by narrowing the search range.
A camera for a conference system generally uses a wide angle lens (whose angle of view is greater than 60 degrees) that causes great image distortion, and is therefore greatly affected by the above problems. The same can be said to an around-view monitor camera.